What does the "blend" value do on a spotlight in cycles?
TLDR Reduce the size of the lamp (something around 0.01 BU).




The blend value still does what it always did in the blender internal render, make the edges of the spotlight softer.  

The reason you can't see much difference when you change the blend value is because of the lamp's size. (Independent from the blend value if you change the lamp's size the light and the shadows will get softer, or sharper.)

Simply set the lamp's size to something very small (around 0.01 BU). Not the angle of the beam, but the size of the lamp (top "Size" field). Now you can adjust the blend value. There isn't that better.

In the picture below both lights have the blend set to 0, but the size is different 1 BU for the pink light, and 0.01 BU for the green light.


For additional reading the cycles documentation for the spotlight is rather short, we have to go to the BI side of the docs to get more info on the blend setting.